Chemical kinetics
Outline
• Reaction rates
• Rate laws
• Integrated rate laws
• Half life
• Arrhenius equation
• Mechanism
• Collision model
• Transition state theory
Kinetics
• Chemical Kinetics is the study of the rate at
which a chemical process occurs.
• Rate –change in some variable per unit time
rate = f ([A], [B], [Y], [I], [C], T, p, . . .)
• Reaction rate –change in concentration per
unit time; M/s
Kinetics
• Thermodymanics - possibility
• Kinetics – how fast...
• Besides information about the speed at which
reactions occur, kinetics also sheds light on the
reaction mechanism (exactly how the reaction
occurs).
• time scale 10-15 s – 1010 s
in laboratory 10-6 s – 105 s
Reaction rate
A + B → C
Stoichiometry
Rate of reaction
Rate of
disappearence
of A
Rate of
appearence
of C
Reaction rate
2A + B → 3C + D
aA + bB → cC + dD
Generally:
Comparing concentrations of
chemicals in a reaction
Types of reaction rate
• Instantaneous rate –rate at a specific time
• Initial rate –instantaneous rate at t= 0
• Average rate –∆[A] over a specific time interval
Reaction Rates
Rate law
Rate Law: equation describing the relationship between the reaction
rate and concentration of a reactant or reactants
Rate = k[A] α[B] β
α – order of reaction with the respect to A
β- order of reaction w.r.t. B
k - rate constant
• Although the exponents in the rate law are often the same as the coefficients
in the balanced reaction, this is not necessarily the case.
• Reaction orders and, thus, rate laws must be determined EXPERIMENTALLY!!!
they cannot be found by looking at the equation, i.e., the stoichiometry of the
reaction
• Overall order = sum of individual order
Reaction order
A→ products
Zero order reaction
First order reaction
Second order reaction
A+B→ products
Reaction order
Reaction order
Factors That Affect Reaction Rates
– Structure, elements, behavior
Basic factors :
– Nature of reactants
– Concentrations of reactants
– Temperature
– Presence of catalysts
– Surface area
Nature of reactants
• Ions react rapidly:
Ag+ + Cl- ↔AgCl(s) Very fast
• Reactions which involve bond breaking are slower:
NH4
+ + OCN- ↔ OC(NH2)2
• Redox reactions in solutions are slow
• Transfer of electrons are faster than those of atoms.
• Reactions between covalently bonded molecules are
slow:
• 2 HI(g) ↔ H2(g) + I2(g)
Factors That Affect Reaction Rates
Factors That Affect Reaction Rates
Concentration of Reactants
• As the concentration of reactants increases, so
does the likelihood that reactant molecules
will collide.
Factors That Affect Reaction Rates
Temperature
• At higher temperatures, reactant molecules
have more kinetic energy, move faster, and
collide more often and with greater energy
Presence of a Catalyst
• Catalysts speed up reactions by changing the
mechanism of the reaction.
• Catalysts are not consumed during the course
of the reaction.
Factors That Affect Reaction Rates
Factors That Affect Reaction Rates
Surface Area
• The rate of reacton increae when the size of the
particles is smaller → The total surface area that is
exposed to collision between particles are larger →
The collision frequency will increase →The rate of
reaction also increases
Rate theory
The Arrhenius Equation
Temperature dependence of the Rate Constant
Most reactions speed up as temperature increases.
(example: food spoils when not refrigerated.)
k =f(T)
Where “f” is some function.
Temperature and rate
Generally, as temperature increases,
so does the reaction rate.
This is because k is temperature
dependent.
The magnitude of a first order rate constant is seen to
increase exponentially with an increase in temperature.
Therefore one can conclude that:
k(T)∝ e f(T)
The Collision Model
• In a chemical reaction, bonds are broken and new
bonds are formed.
• Molecules can only react if they collide with each
other
• Furthermore, molecules must collide with the correct
orientation and with enough energy to cause bond
breakage and formation.
Activation Energy
• In other words, there is a minimum amount of energy required for
reaction: the activation energy, Ea
• Just as a ball cannot get over a hill if it does not roll up the hill with
enough energy, a reaction cannot occur unless the molecules
possess sufficient energy to get over the activation energy barrier.
Transition State Theory
• This is a reaction where methyl isonitrileis
rearranging to acetonitrile
• This reaction passes through an intermediate
state in which the N C is sitting sideways.
Reaction Coordinate Diagram
Maxwell – Boltzmann distribution
• As the temperature
increases, the curve
flattens and
broadens.
• Thus at higher
temperatures, a larger
population of
molecules has higher
energy
• Fraction of molecules possessing Ea or greater
• number of collisions per second
• fraction of molecules having the correct
orientation
Arrhenius Equation
A - frequency factor
- a number that represents the likelihood that collisions
would occur with the proper orientation for reaction.
Arrhenius Equation
Arrhenius Equation